This invention relates to frequency synthesizers for wireless communications equipment and more particularly to such devices that include provisions for assuring the frequency synthesizer is generating the desired frequency prior to allowing, for example, the transmission of a radio frequency carrier.
Modern day wireless communications equipment often need the capability to routinely, rapidly, and reliably change between any two of, for example, a large number of potential radio frequency carriers with specific system critical parameters. This capability makes it possible to utilize efficient, high capacity, wireless communications technologies such as cellular telephone or trunked radio systems. This type of system, in turn, makes it feasible to satisfy, to a greater extent, the exploding demands of the wireless communications markets with a largely fixed frequency spectrum allocation.
The frequency synthesizer has enjoyed widespread usage and provides an economically effective way of generating, and rapidly changing between any two of, a large number of radio frequency carriers with desired parameters. To generate a specific radio frequency carrier, the frequency synthesizer must be programmed with an instruction corresponding to the desired radio frequency carrier. Some time after being programmed, depending on its dynamic characteristics, the frequency synthesizer should "LOCK" to the desired radio frequency carrier. To change the radio frequency carrier, the synthesizer must be reprogrammed, and allowed to "LOCK", to the new frequency. During this time the frequency synthesizer will be generating undesired, i.e., unauthorized, and often system-detrimental carrier frequencies. In an effort to avoid some problems associated with the above, prior devices simply presume the synthesizer has been correctly programmed and utilize an out-of-lock indication to preclude transmission until the synthesizer is actually locked.
This, however, may well present a problem. If the programming accuracy presumption is in fact incorrect but nevertheless the synthesizer locks to a frequency, the equipment may be allowed to transmit on either an unauthorized frequency or, almost certainly, a system-detrimental frequency. This situation could occur in any number of ways, including, a temporary or permanent hard fault in the programming system, or a soft fault such as, controller software errors or other possibly indeterminate, intermittent causes. The net effect of these problems is almost certainly communications failure for the user of the faulty piece of equipment and very likely other users, if the equipment is allowed to transmit on an undesired frequency. Indirectly, burdens may be imposed on all users while the system attempts to compensate for the errant equipments behavior.
To eliminate part of these problems, such as a permanent hard fault, known prior devices have used an approach whereby the synthesizer is first programmed to a non-useful frequency, allowed to lock, and then programmed to the desired frequency. By monitoring an out-of-lock detector for an "unlock," "lock," "unlock," and finally "LOCK" sequence of indications, a reasonable implication that the programming system is functioning, at least in part, can be made. This approach, while helpful, does not protect against soft or other indeterminate errors and is completely contrary to the requirement of rapidly changing to the new frequency.
Today's system requirements, routinely changing operating frequencies with flexible parameters, have dramatically increased the amount of information which must be successfully programmed. This only exacerbates an already potentially serious problem.
From the above it will be appreciated that a need exists for a frequency synthesizer programming system which provides enhanced assurance that the frequency synthesizer is locked on the desired frequency at all times and under all operating conditions.